Semiconductor light-emitting device with quantum well

ABSTRACT

A semiconductor light-emitting device of Group III-V compound semiconductors includes a quantum well layer, which is formed over a substrate and includes a barrier layer and a well layer that are alternately stacked one upon the other. The band gap of the well layer is narrower than that of the barrier layer. The well layer contains indium and nitrogen, while the barrier layer contains aluminum and nitrogen. In this structure, a tensile strain is induced in the barrier layer, and therefore, a compressive strain induced in the quantum well layer can be reduced. As a result, a critical thickness, at which pits are created, can be increased.

BACKGROUND OF THE INVENTION

The present invention relates to a light-emitting device like asemiconductor laser device, and more particularly relates to asemiconductor light-emitting device for emitting radiation in theultraviolet to blue regions. The present invention also relates to amethod for fabricating the semiconductor light-emitting device and to anoptical disk apparatus using the light-emitting device.

In recent years, semiconductor light-emitting devices that can emitradiation at short wavelengths ranging from the ultraviolet to blueregions, or semiconductor laser devices, in particular, have beenresearched and developed vigorously. This is because such light-emittingdevices are expected to further increase the recording density ofoptical disks or the resolution of laser printers and are applicable tooptical measuring instruments, medical equipment, display devices,illuminators and so on.

Examples of semiconductor materials that can emit radiation at suchshort wavelengths include Group III nitride semiconductors. Forinstance, a semiconductor laser device with a multiple quantum wellactive layer, which is a stack of silicon (Si)-doped GaInN/GaInN layers,can oscillate continuously at a wavelength of about 401 nm and at roomtemperature and can operate for as long as about 3,000 hours under theconditions that the ambient temperature is 20° C. and the output powerthereof is 2 mW. See Japanese Journal of Applied Physics, Vol. 36(1997), pp. 1568-1571, for example.

Group III nitride semiconductor crystals are generally grown by ametalorganic vapor phase epitaxy (MOVPE) process. For example, JapaneseLaid-Open Publication No. 6-196757 discloses a method of growing asemiconductor layer of GaInN of excellent crystal quality on asemiconductor layer of GaN by using nitrogen as a carrier gas.

The known method of producing a Group III nitride semiconductor,however, is disadvantageous in that pits are created in the GaInN/GaNmultiple quantum well structure thereof (to be an active layer) at ashigh a density as 10⁸ to 10⁹ cm⁻² as described in Applied PhysicsLetters, Vol. 72 (1998), pp. 710-712, for example.

Those pits adversely affect the operation characteristics of alight-emitting device, e.g., raises the threshold value, at which thelaser device starts to oscillate, or lowers the reliability thereof.This is because the existence of the pits not only decreases theluminous efficacy, but also causes localized levels by making thecomposition of In non-uniform, constitutes a source of diffusion of Inbeing grown or results in scattering or absorption loss in an opticalwaveguide.

To obtain a Group III nitride semiconductor light-emitting device, orsemiconductor laser device, in particular, with characteristicspractically applicable to an optical disk apparatus, for example, thecomposition of In within the GaInN well layer thereof should beuniformized. In addition, each multiple quantum well layer should be ofuniform quality and be sufficiently planarized.

Moreover, the structure of the device should be modified such thatelectrons, which are injected from an n-type conductive layer into thequantum well layer, can be injected into the active layer efficientlyand uniformly without overflowing into a p-type conductive layer duringthe operation of the device.

SUMMARY OF THE INVENTION

An object of the present invention is solving the problems of the priorart to suppress the creation of pits in quantum well layer containingindium and nitrogen in a Group III nitride semiconductor light-emittingdevice and to inject electrons into the quantum well layer moreefficiently.

To achieve this object, the present invention takes the followingmeasures:

1) Each barrier layer included in the quantum well layer containsaluminum;

2) The stress vector of each barrier layer is of the opposite sign tothat of each well layer;

3) Only one of barrier layers that is in contact with a p-typeconductive layer contains aluminum in multiple quantum well layer;

4) In growing the quantum well layer by an MOVPE process,triethylgallium is used as a gallium source.

The present inventors analyzed how inverted hexagonal parallelepipedpits with {1-101} planes as facets are formed at a high density in aGaInN/GaN or GaInN/GaInN multiple quantum well structure in accordancewith a conventional fabrication process. As a result, we reached thefollowing conclusions.

To relax a compressive strain induced in a GaInN layer or a strainresulting from localized segregation of In, nuclei of pits are createdat more than a critical thickness. In addition, at the growthtemperature of the GaInN layer (usually at a growth temperature of about800° C.), the growth rate for {1-101} planes is lower than that for the(0001) plane in the GaInN layer. Accordingly, as the crystals aregrowing, the pits are also increasing their sizes. Those pits, whichhave been created in the GaInN layer, are gradually filled in and thesurfaces of the crystals are planarized while an optical guide layer, acladding layer and so on are grown one upon the other on the GaInN layerat a growth temperature of about 1000° C. This is because the growthrate for the {1-101} planes is higher than that for the (0001) plane inthe optical guide layer, etc.

It should be noted that when a zone axis index or a Miller indexrepresenting a crystallographic plane orientation is followed by anegative sign, the index following the negative sign is a negativedirection index in this specification.

The present inventors examined various methods of suppressing thecreation of those pits. As a result, we made the following findings.

Specifically, if the multiple quantum well structure includes analuminum (Al)-containing barrier layer, then a tensile strain is inducedin the barrier layer, and a compressive strain applied to the multiplequantum well structure decreases. Consequently, the critical thicknessincreases.

In addition, the existence of Al with high electric field intensity in acrystal reduces the diffusion of In, thus suppressing the segregation ofIn, which strongly tends to segregate locally.

Moreover, the growth rate of the Al-containing semiconductor layer,i.e., an AlGaN layer, for the {1-101} planes is not so different fromthat for the (0001) plane compared to the GaInN layer. Accordingly, theexpansion of pits can be reduced.

Furthermore, if the In mole fraction in the well layer is 0.1 or less,the total thickness of the multiple quantum well structure does notexceed the critical thickness.

Furthermore, if the strain vector of the barrier layer is of the signopposite to that of well layer, then the total strain applied to themultiple quantum well structure can be reduced, thus increasing thecritical thickness.

Also, if triethylgallium (TEG) is used a gallium source in forming themultiple quantum well structure, then the growth rate for the (0001)plane is not so different from that for the {1-101} planes in thequantum well structure.

Accordingly, the expansion of pits can be reduced.

As for a method for injecting electrons more efficiently, we made thefollowing findings.

If the multiple quantum well structure includes a barrier layer with astrain vector of the opposite sign to that of each well layer, then thetotal strain induced in the multiple quantum well structure can besmaller. Thus, the intensity of a piezoelectric field induced in themultiple quantum well structure decreases. As a result, electrons areinjected into the well layers more uniformly.

Alternatively, if only one barrier layer in contact with a p-typeconductive layer contains Al and the other barrier layers, which are notin contact with the p-type conductive layer, do not contain Al, then theelectrons injected into the well layers do not overflow into the p-typeconductive layer. As a result, the electrons can be injected into thewell layers more efficiently.

Specifically, a first semiconductor light-emitting device according tothe present invention is made of Group III-V compound semiconductors.The device includes a quantum well layer, which is formed over asubstrate and includes a barrier layer and a well layer that arealternately stacked one upon the other. The band gap of the well layeris narrower than that of the barrier layer. And the well layer containsIn and N, while the barrier layer contains Al and N.

In the first semiconductor light-emitting device, the barrier layercontains Al and N. That is to say, if Al is contained in the barrierlayer, a tensile strain is induced in the barrier layer to fill in thepits, which are created at more than the critical thickness to relax thecompressive strain induced in the well layer. As a result, thecompressive strain induced in the quantum well layer decreases and thecritical thickness increases. Also, since Al is contained in the barrierlayer, the In segregation in the well layer can be suppressed. Moreover,the growth rate for the {1-101} planes is not so different from that forthe (0001) plane compared to a well layer containing In. Accordingly,the expansion of pits can be suppressed, thus reducing the thresholdcurrent of the light-emitting device and greatly improving thereliability of the device.

In the first semiconductor light-emitting device, a plurality of thebarrier layers are preferably provided between p- and n-type conductivelayers. One of the barrier layers that is in contact with the p-typeconductive layer preferably has an aluminum mole fraction larger thanthat of the other barrier layer(s) that is/are not in contact with thep-type conductive layer. In such an embodiment, since Al is added to thebarrier layer in contact with the p-type conductive layer, the barrierlayer has its heterobarrier increased. Thus, it is possible to preventelectrons, which have been externally injected, from overflowing intothe p-type conductive layer without being injected into the well layers.As a result, the electrons can be injected into the well layers moreefficiently.

In this particular embodiment, the aluminum mole fraction of the onebarrier layer in contact with the p-type conductive layer preferablyincreases from a part thereof closest to the n-type conductive layertoward another part thereof closest to the p-type conductive layer. Insuch an embodiment, the hole density in the barrier layer in contactwith the p-type conductive layer can be decreased, thus increasing theefficiency with which holes are injected into the well layers.

In the first semiconductor light-emitting device, the well layer ispreferably made of gallium indium nitride (GaInN) or aluminum galliumindium nitride (AlGaInN), while the barrier layer is preferably made ofaluminum gallium nitride (AlGaN). In such an embodiment, the creation ofthe pits can be suppressed with much more certainty in the quantum welllayer.

A second semiconductor light-emitting device according to the presentinvention is made of Group III-V compound semiconductors. The deviceincludes a quantum well layer, which is formed over a substrate andincludes a barrier layer and a well layer that are alternately stackedone upon the other. The band gap of the well layer is narrower than thatof the barrier layer. The barrier layer has a strain vector of a signopposite to that of a strain vector of the well layer.

In the second semiconductor light-emitting device, since the strainvectors of the barrier and well layers are of mutually opposite signs,the strain quantities in the quantum well structure are canceled by eachother and decrease. Accordingly, the critical thickness, at which pitsare created, increases and in addition, the piezoelectric field inducedin the quantum well structure decreases. As a result, electrons andholes are injected into each well layer uniformly, thus increasing theluminous efficacy.

In the second semiconductor light-emitting device, the well layerpreferably contains In and the barrier layer preferably contains Al.

The first or second semiconductor light-emitting device preferablyfurther includes first and second optical guide layers. The firstoptical guide layer is provided on one side of the quantum well layerthat is closer to the substrate, while the second optical guide layer isprovided on another side of the quantum well layer that is opposite tothe substrate. The band gap of the barrier layer is preferably smallerthan or equal to that of the first and second optical guide layers.

Also, an In mole fraction of the well layer is preferably larger than 0and equal to or smaller than 0.1. In such an embodiment, it is possibleto prevent the total thickness of the quantum well layer from exceedingthe critical thickness. As a result, the creation of the pits can besuppressed with much more certainty in the quantum well layer.

Moreover, the barrier layer or the well layer preferably containssilicon (Si) as a dopant. In such an embodiment, it is possible toprevent In from locally segregating in the quantum well layer, thusrelaxing the strain resulting from such local segregation of In. As aresult, the creation of the pits, which are formed to reduce the strain,can be suppressed.

A third semiconductor light-emitting device according to the presentinvention is made of Group III-V compound nitride semiconductors. Thedevice includes: a quantum well layer, which is formed over a substrateand includes a plurality of barrier layers and a well layer that arealternately stacked one upon the other, the band gap of the well layerbeing narrower than that of each said barrier layer; and p- and n-typeconductive layers formed over the substrate to vertically interpose thequantum well layer therebetween. One of the barrier layers that is incontact with the p-type conductive layer contains aluminum, while theother barrier layer(s) that is/are not in contact with the p-typeconductive layer contain(s) no aluminum.

In the third semiconductor light-emitting device, a large heterobarrier,which is caused by the addition of Al, exists between the barrier layerin contact with the p-type conductive layer and the well layer incontact with the barrier layer on the opposite side to the p-typeconductive layer. Thus, it is possible to prevent electrons from goingover the well layer to overflow into the p-type conductive layer. Inaddition, since a piezoelectric field is induced in the direction inwhich the overflow of the electrons over the well layer is suppressed,the electrons can be injected into the well layer more efficiently.

In the third semiconductor light-emitting device, the well layerpreferably contains In.

In the third semiconductor light-emitting device, the well layer ispreferably made of GaInN, the barrier layer in contact with the p-typeconductive layer is preferably made of AlGaN, and the other barrierlayer(s) is/are preferably made of GaInN or GaN.

An inventive method for fabricating a semiconductor light-emittingdevice is adapted to fabricate a semiconductor light-emitting device ofGroup III-V compound semiconductors, in which a quantum well layerincluding barrier and well layers is formed by a metalorganic vaporphase epitaxy process over a substrate by alternately stacking thebarrier and well layers one upon the other. The band gap of the welllayer is narrower than that of the barrier layer. The method includesthe steps of: forming the barrier layer, which contains gallium (Ga) andnitrogen (N), over the substrate by using at least gallium and nitrogensources as first source materials; and forming the well layer, whichcontains Ga, In and N, on the barrier layer by using at least gallium,indium and nitrogen sources as second source materials. The galliumsource used in the steps of forming the barrier layer and the well layeris triethylgallium (TEG).

According to the inventive method for fabricating a semiconductorlight-emitting device, TEG is used as the gallium source in the steps offorming the barrier layer and the well layer. Thus, the growth rate ofthe quantum well layer for the {1-101} planes is not so different fromthat for the (0001) plane. Accordingly, the expansion of pits can besuppressed, thus improving the crystal quality of the well layers. As aresult, the operating performance of the device can be improved.

In the inventive method for fabricating a semiconductor light-emittingdevice, the step of forming the barrier layer preferably includes thestep of forming the barrier layer of AlGaN by using an aluminum sourceas an additional one of the first source materials. The step of formingthe well layer preferably includes the step of forming the well layer ofeither GaInN or AlGaInN by using an aluminum source as an additional oneof the second source materials. Also, an In mole fraction of the welllayer is preferably larger than 0 and equal to or smaller than 0.1.

An optical disk apparatus according to the present invention includes:the semiconductor light-emitting device according to any of the firstthrough aspects of the present invention; a condensing optical systemfor condensing outgoing radiation, which has been emitted from thesemiconductor light-emitting device, on a storage medium on which datahas been recorded; and a photodetector for receiving light that has beenreflected from the storage medium.

In the inventive optical disk apparatus, the photodetector preferablyreads the data that has been recorded on the storage medium based on areflected part of the outgoing radiation.

In this case, the photodetector is preferably provided near thesemiconductor light-emitting device.

In this particular embodiment, the photodetector is preferably providedon a principal surface of a support member made of silicon, and thesemiconductor light-emitting device is preferably supported on theprincipal surface of the support member.

In such a case, the principal surface of the support member ispreferably provided with a concave portion with a micro mirror on asidewall thereof. And the semiconductor light-emitting device ispreferably secured to the bottom of the concave portion of the supportmember such that the outgoing radiation emitted from the semiconductorlight-emitting device is reflected from the micro mirror and advancessubstantially vertically to the principal surface of the support member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a schematic structure of asemiconductor light-emitting device according to a first embodiment ofthe present invention.

FIG. 2 is a cross-sectional view illustrating a detailed structure of amultiple quantum well active layer in the semiconductor light-emittingdevice according to the first embodiment of the present invention.

FIG. 3 is a graph illustrating a relationship between the In molefraction in the well layers and the oscillation threshold current in thesemiconductor light-emitting device according to the first embodiment ofthe present invention.

FIG. 4 is an energy band diagram illustrating the band gaps in thesemiconductor light-emitting device according to the first embodiment ofthe present invention.

FIG. 5 is an energy band diagram illustrating the band gaps in acomparative semiconductor light-emitting device with a barrier layercontaining no aluminum.

FIG. 6 is a cross-sectional view illustrating a detailed structure of amultiple quantum well active layer in the semiconductor light-emittingdevices according to first and second modified examples of the firstembodiment of the present invention.

FIG. 7 is an energy band diagram illustrating the band gaps in thesemiconductor light-emitting device according to the first modifiedexample of the first embodiment of the present invention.

FIG. 8 is an energy band diagram illustrating the band gaps in thesemiconductor light-emitting device according to the second modifiedexample of the first embodiment of the present invention.

FIG. 9 is a cross-sectional view illustrating a schematic structure of asemiconductor light-emitting device according to a second embodiment ofthe present invention.

FIG. 10 is a cross-sectional view illustrating a detailed structure ofthe multiple quantum well active layer in the semiconductorlight-emitting device according to the second embodiment of the presentinvention.

FIG. 11 is an energy band diagram illustrating the band gaps in thesemiconductor light-emitting device according to the second embodimentof the present invention.

FIG. 12 illustrates a schematic arrangement of an optical disk apparatusaccording to a third embodiment of the present invention.

FIG. 13 illustrates a schematic arrangement of an optical disk apparatusaccording to a fourth embodiment of the present invention.

FIG. 14 is a perspective view illustrating a laser unit in the opticaldisk apparatus according to the fourth embodiment of the presentinvention.

FIG. 15 is a cross-sectional view illustrating a schematic arrangementof a hologram in the optical disk apparatus according to the fourthembodiment of the present invention.

FIG. 16(a) is a plan view schematically illustrating a laser unit in theoptical disk apparatus according to the fourth embodiment of the presentinvention; and

FIGS. 16(b) through 16(d) are plan views schematically illustrating howphotodiodes of the laser unit in the optical disk apparatus according tothe fourth embodiment of the present invention receive light to beconverted into signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Hereinafter, a first embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 illustrates a schematic cross-sectional structure of asemiconductor light-emitting device according to the first embodiment ofthe present invention. On a GaN substrate 11 with a (0001) plane as itsprincipal surface, for example, a buffer layer 12 of n-type GaN, ofwhich the upper surface is partially exposed; a cladding layer 13 ofn-type AlGaN for creating a potential barrier for a multiple quantumwell active layer (to be described below) and thereby confining n-typecarriers therein; an optical guide layer 14 of n-type AlGaN forconfining the radiation created therein; a multiple quantum well activelayer 15, which is formed by alternately stacking GaInN well layers andAlGaN barrier layers one upon the other and in which n- and p-typecarriers are recombined with each other to emit radiation therefrom; anoptical guide layer 16 of p-type AlGaN for confining the radiationproduced therein; a cladding layer 17 of p-type AlGaN for creating apotential barrier for the multiple quantum well active layer 15 andthereby confining p-type carriers therein; a current blocking layer 18of n-type GaN for injecting current into the multiple quantum wellactive layer 15 efficiently; and a contact layer 19 of p-type GaN formaking ohmic contact with a p-side electrode 20 are formed in thisorder.

The p-side electrode 20 of Ni/Au is formed on the upper surface of thecontact layer 19. On the exposed part of the buffer layer 12, formed isan n-side electrode 21 of Ti/Al.

In this case, materials for the substrate 11 are not limited to galliumnitride (GaN), but include sapphire (Al₂O₃), silicon carbide (SiC),silicon (Si), spinel, zinc sulfide (ZnS), zinc oxide (ZnO) and galliumarsenide (GaAs), for example. If the substrate 11 is made of a materialother than GaN, however, another buffer layer of GaN, for example,should be deposited at a low temperature between the substrate 11 andthe buffer layer 12 to obtain quality GaN semiconductor crystals bybuffering a lattice misfit between the substrate 11 and the crystals,because the substrate 11 and the GaN semiconductor crystals are ofdissimilar materials.

The plane orientation of the substrate 11 does not have to be such aplane with a low index. Alternatively, the zone axis thereof may beinclined in a predetermined direction. For example, where the substrateis made of GaN, the zone axis of the substrate may be inclined by 2degrees in the [11-20] direction about a (0001) plane of GaN. Theconductivity type of the substrate 11 may be either n-type or p-type oreven an insulating substrate may also be used.

By forming the GaN buffer layer 12 on the GaN substrate 11, respectivesemiconductor layers, which will constitute a quality Group III nitridesemiconductor laser structure, can be grown according to thisembodiment. GaN is selected for the buffer layer 12 because crystals ofquality can be obtained more easily than any other Group III nitridesemiconductor. The thickness of the buffer layer 12 has only to be about100 nm or more. When the substrate 11 is not made of a material with lowresistance as is done in this embodiment, the p- and n-side electrodes20 and 21 both need to be formed on the circuitry side of the substrate11. Thus, the thickness herein should be at least about 1000 nm or more.

The n- and p-type cladding layers 13 and 17 of AlGaN both have an Almole fraction (composition) of 0.09. The thicknesses of the n- andp-type cladding layers 13 and 17 are preferably about 900 nm and about600 nm, respectively. A tensile strain is applied to AlGaN. Accordingly,as the Al mole fraction or the thicknesses of these cladding layers 13and 17 increase, cracking is more and more likely to be caused duringthe crystal growth thereof. Thus, to avoid such cracking, a strainedsuperlattice structure is preferably formed by alternately stackingAl_(0.18)Ga_(0.82)N layers (thickness: about 3 nm) of a first group andGaN layers (thickness: about 3 nm) of a second group one upon the other.Also, a so-called “modulated doping” may be adopted in such a case. Thatis to say, the first group of layers may be doped with an n- or p-typedopant while the second group of layers may be non-doped. Moreover, byusing AlGaInN quaternary crystals, a laser structure thatlattice-matches with the GaN substrate 11 can be formed. As a result,not only cracking but also dislocations can be suppressed.

The n- and p-type optical guide layers 14 and 16 of AlGaN both have anAl mole fraction of 0.02 and a thickness of about 100 nm, for example.The light confinement function of a laser structure is also determinedby the specifications (such as thicknesses and refractive indices) ofthe multiple quantum well active layer 15 and the n- and p-type claddinglayers 13 and 17. Thus, as the case may be, the optical guide layers 14and 16 may be made of GaN not containing Al.

Hereinafter, the multiple quantum well active layer characteristic ofthe first embodiment will be described with reference to theaccompanying drawings. FIG. 2 illustrates a detailed cross-sectionalstructure of the multiple quantum well active layer 15 according to thefirst embodiment. As shown in FIG. 2, the multiple quantum well activelayer 15 according to this embodiment includes: three GaInN well layers151, each of which is about 3 nm thick and has an In mole fraction of0.08; two AlGaN barrier layers 152, each of which is formed between anassociated pair of well layers 151, is about 5 nm thick and has an Almole fraction of 0.02; and one protective layer 153, which is formed onthe upper surface of the third well layer 151, is about 5 nm thick andhas an Al mole fraction of 0.15. In this case, the protective layer 153is provided to prevent In from being desorbed again from the GaInNcrystals in the uppermost well layer 151 into the gas after the welllayers 151 and barrier layers 152 have been grown. The protective layer153 is also provided to inject electrons into the active layerefficiently during the operation of the light-emitting device. Theprotective layer 153 is preferably doped with a p-type dopant.

The oscillation wavelength of the laser radiation is controllable, orshortened or lengthened, depending on the In mole fraction in each welllayer 151. To keep the quality of crystals high, however, the In molefraction should be larger than zero and equal to or smaller than 0.3.Preferably, the In mole fraction should be larger than zero and equal toor smaller than 0.2 to prevent the In composition from gettingnon-uniform and to reduce the threshold current of oscillation in themultiple quantum well active layer 15. More preferably, the In molefraction should be larger than zero and equal to or smaller than 0.1 tosuppress the creation of pits during the crystal growth. Also, the welland barrier layers 151 and 152 are preferably doped with Si as a dopant.

FIG. 3 illustrates a relationship between the In mole fraction of thewell layers and the oscillation threshold current in a semiconductorlaser diode, in which the length of the resonant cavity thereof is about1 mm and the width of the ridge is about 5 μm. As can be seen from FIG.3, where the In mole fraction of the well layers 151 is in the rangefrom 0.05 to 0.1, the oscillation threshold current is smaller than 200mA.

As described above, the creation of pits during the crystal growth canbe suppressed in the multiple quantum well active layer 15 according tothis embodiment. In addition, electrons and holes can be injected intothe respective well layers 151 more uniformly and electrons can beinjected more efficiently.

Firstly, since Al is contained in the barrier layers 152 in the multiplequantum well active layer 15, a tensile strain is induced in the barrierlayers 152 in such a direction as reducing the compressive strainapplied to the well layers 151. Accordingly, the critical thickness, atwhich the pits are created, can be increased. That is to say, so long asthe total thickness of the multiple quantum well active layer 15 iswithin the increased critical thickness, the creation of pits can besuppressed effectively.

Secondly, the existence of Al with high electric field intensity in thecrystals minimizes the diffusion of In, thus suppressing the segregationof In, which strongly tends to segregate locally. Moreover, the growthrate for the {1-101} planes is not so different from that for the (0001)plane in the AlGaN barrier layer 152 compared to the GaInN well layer151. Accordingly, the expansion of pits can be reduced.

FIG. 4 is a band diagram illustrating the band gaps of the semiconductorlight-emitting device according to this embodiment in view of thepiezoelectric field. FIG. 5 is a band diagram illustrating the band gapsof a comparative semiconductor light-emitting device including GaInNbarrier layers. In FIGS. 4 and 5, energy regions corresponding to therespective semiconductor layers shown in FIGS. 1 and 2 are identified bythe same reference numerals. As can be seen, the piezoelectric field,which is strongly induced over the entire multiple quantum well activelayer 15 in a direction heading toward the lower right in FIG. 5, hasbeen weakened in FIG. 4 because an electric field has been induced inthe Al-containing barrier layers 152 in the direction heading toward theupper right. Accordingly, electrons and holes can be injected into therespective well layers 151 more uniformly and the luminous efficacyimproves as a result.

The Al mole fraction of the barrier layers 152 is larger than zero andequal to or smaller than that of the n- and p-type cladding layers 13and 17 since a trade-off is necessary between suppression of pits anduniform injection of electrons and holes into the well layers.Preferably, the Al mole fraction of the barrier layers 152 is largerthan zero and equal to or smaller than that of the n- and p-type opticalguide layers 14 and 16. That is to say, the band gap of the barrierlayers 152 is approximately equal to or smaller than that of the n- andp-type optical guide layers 14 and 16.

Also, since no In is contained in the barrier layers 152 of the multiplequantum well active layer 15, the compressive strain induced in themultiple quantum well active layer 15 can be reduced. In addition, Inexists only in the well layers 151, and therefore, it is possible toprevent In from diffusing. That is to say, the expansion of a continuousIn segregated region can be avoided.

Moreover, by setting the In mole fraction in the well layers 151 at 0.1or less, it is possible to prevent the total thickness of the multiplequantum well active layer 15 from exceeding the critical thickness.

In this case, the critical thickness is not the only factor determiningthe thickness and number of the well layers 151. In view of luminousefficacy, the thickness of each of the well layers 151 is preferably inthe range from about 2 to about 4 nm. Also, two, three or four welllayers 151 are preferably formed to inject carriers uniformly and toattain a sufficient gain.

Furthermore, if the crystal being grown in the multiple quantum wellactive layer 15 are doped with Si as a dopant, the segregation of In canbe suppressed and luminous efficacy improves. A specific mechanismthereof is not clear, but it is probably due to the surfactant effectsattained by Si.

As described above, the barrier layers 152 in the multiple quantum wellactive layer 15, consisting of the well and barrier layers 151 and 152,is made of AlGaN crystals in the semiconductor light-emitting deviceaccording to this embodiment. In this structure, a tensile strain isinduced in the barrier layers 152 and a compressive strain applied tothe multiple quantum well active layer 15 can be reduced, thusincreasing the critical thickness of crystals.

Also, by setting the In mole fraction in the well layers 151 composed ofGaInN crystals at 0.1 or less, it is possible to prevent the totalthickness of the multiple quantum well active layer 15 from exceedingthe critical thickness, at which the pits are created. Accordingly, ifthe total thickness of the multiple quantum well active layer 15 iswithin the increased critical thickness, the creation of pits can besuppressed very effectively.

Hereinafter, a method for fabricating the semiconductor light-emittingdevice with the above structure will be described with reference toFIGS. 1 and 2. In the following description, a procedure of fabricatingthe semiconductor light-emitting device of this embodiment by an MOVPEprocess will be exemplified.

In the MOVPE process, alkyl metal compounds are used as source materialsof Group III elements. Specifically, trimethylgallium (TMG) ortriethylgallium (TEG) is used as a gallium source of the Group IIIelements. Trimethylaluminum (TMA) is used as an aluminum source. Andtrimethylindium (TMI) or ethyldimethylindium is used as an indiumsource.

Ammonium (NH₃) or hydrazine (N₂H₄) is used as source material of theGroup V element (i.e., nitrogen). Silane (SiH₄) gas is used as a siliconsource for supplying an n-type dopant. And bis-cyclopentadienylmagnesium (Cp₂Mg) is used as a magnesium source for supplying a p-typedopant.

First, the GaN substrate 11 with a (0001) plane as its principal surfaceis cleaned and then placed on a susceptor within a reaction chamber.Then, after the reaction chamber has been evacuated, the substrate 11 isheated at 1030° C. for 10 minutes within hydrogen and ammonium ambientat a pressure of about 800×10² Pa (=600 Torr), thereby cleaning thesurface of the substrate 11.

Next, the temperature of the substrate is set to 1000° C. and then TMGand ammonium are supplied into the reaction chamber at a mole ratio ofammonium supplied as a Group V element to TMG supplied as a Group IIIelement (hereinafter, simply referred to as “V/III mole ratio”) of about5000. At the same time, silane gas, which has been diluted with nitrogenas an Si dopant, is also supplied. In this manner, the n-type GaN bufferlayer 12 with a carrier density of 8×10¹⁷/cm⁻³ is deposited to be about2500 nm thick on the principal surface of the substrate 11 as shown inFIG. 1. In this process step, the growth rate is about 25 nm/min.

Next, TMA is newly supplied as an aluminum source onto the buffer layer12, thereby growing the n-type Al_(0.1)Ga_(0.9)N cladding layer 13 to athickness of about 900 nm. Subsequently, the n-type Al_(0.02)Ga_(0.98)Noptical guide layer 14 is grown thereon to a thickness of about 100 nm.In this process step, since the solid phase ratio of Al is approximatelyequal to the vapor phase ratio thereof, the Al mole fraction in AlGaN iseasily controllable.

Subsequently, as shown in FIG. 2, the substrate is cooled down to atemperature of about 800° C. Then, TEG, TMA and TMI are supplied asGroup III sources, ammonium is supplied as a Group V source and nitrideis supplied as a carrier gas. In this manner, the multiple quantum wellactive layer 15, which is a multilayer structure consisting of theGa_(0.92)In_(0.08)N well layers 151 (thickness: about 3 nm),Al_(0.02)Ga_(0.98)N barrier layers 152 (thickness: about 5 nm) and anAl_(0.15)Ga_(0.85)N protective layer 153, is grown on the n-type opticalguide layer 14. In this case, the growth rate of each semiconductorlayer is about 1 nm/min. The source materials for the respectivesemiconductor layers are supplied under the following conditions. In thewell layers 151, the vapor phase ratio of TMI is 0.7 and the V/III ratiois 50,000. In the barrier layers 152, the vapor phase ratio of TMA is0.02 and the V/III ratio is 200,000. In the protective layer 153, thevapor phase ratio of TMA is 0.15 and the V/III ratio is 190,000. Itshould be noted that the protective layer 153 is preferably grown at arelatively high temperature of about 900° C.

The fabrication process according to this embodiment is characterized byusing TEG as a gallium source. Since the decomposition temperature ofTEG is lower than that of TMG, the proportion of Ga atoms, to which noalkyl groups are bonded, is higher on the surfaces of crystals growing.In addition, the surface diffusion length of the Ga atoms, to which noalkyl groups are bonded, is longer than that of Ga molecules to whichalkyl groups are bonded. Moreover, the Ga atoms, to which no alkylgroups are bonded, do not grow selectively so much on the surfaces ofcrystals. Considering these properties, if TEG is used as the galliumsource, then the growth rate for the (0001) plane is not so differentfrom that for the {1-101} planes, thus suppressing the expansion ofpits.

Next, as shown in FIG. 1, the temperature of the substrate is raisedagain up to about 1000° C. Then, TMG and TMA as Group III sources andammonium as a Group V source are supplied into the reaction chamberusing hydrogen as a carrier gas and Cp₂Mg is also supplied thereto as anMg dopant. In this manner, the p-type AlGaN optical guide layer 16 witha thickness of about 100 nm is grown on the multiple quantum well activelayer 15. Subsequently, the p-type AlGaN cladding layer 17 is depositedto be about 600 nm thick on the p-type optical guide layer 16.Thereafter, the supply of the source gases is suspended to lower thetemperature of the substrate 11 to room temperature.

By processing a semiconductor wafer, or an epitaxial substrate, which isobtained through these crystal-growing process steps, in a predeterminedmanner, a singlemode laser device is obtained. Specifically, theepitaxially grown substrate is subjected to photolithography, dryetching, burying layer re-growth, electrode deposition, cleavage andmounting process steps in this order.

First, the photolithographic and dry etching process steps are performedto define a striped SiO₂ mask pattern with a width of about 3 μm on thep-type cladding layer 17. Next, using the mask pattern defined, thep-type cladding layer 17 is dry-etched to a depth of about 500 nm suchthat the p-type cladding layer 17 has ridge portions.

Then, the burying layer re-growth process step is performed.Specifically, the substrate 11 including the p-type cladding layer 17with the ridge portions is loaded into the reaction chamber of the MOVPEapparatus again, thereby selectively growing the n-type GaN currentblocking layer 18 so as to fill in the regions beside the ridge portionsof the p-type cladding layer 17.

Thereafter, the substrate 11 is unloaded from the reaction chamber andthe mask pattern is removed therefrom. Then, the substrate 11 is loadedinto the reaction chamber again to grow the p-type GaN contact layer 19with a carrier density of 8×10¹⁷/cm⁻³ to be about 300 nm thick on thecurrent blocking layer 18, as well as over the ridge portions of thep-type cladding layer 17.

The acceptor Mg, or the p-type dopant introduced into the respectivep-type semiconductor layers, may be activated either within the reactionchamber or within another heat treatment furnace after the substrate 11has been unloaded from the reaction chamber. Also, the heat treatmentmay be conducted at the same time with a sintering process for electrodedeposition. The heat treatment may be conducted at about 600° C. forabout 20 minutes within nitrogen ambient.

Subsequently, in the electrode deposition process step, the p-sideelectrode 20 is selectively formed by an evaporation technique as astack of two types of conductor films, e.g., nickel (Ni) and gold (Au)films, with thicknesses of 10 and about 300 nm, respectively, on part ofthe upper surface of the contact layer 19, which is located over theridge portions of the p-type cladding layer 17.

Next, a mask pattern is defined over a region where the p-side electrodewill be formed and the epitaxial layers are dry-etched using the maskpattern, thereby partially exposing the buffer layer 12. Then, then-side electrode 21 is selectively formed as a stack of two types ofconductor films, e.g., titanium (Ti) and Al films, on the exposed regionby an evaporation technique.

Thereafter, the cleavage and mounting process steps are carried out.Specifically, first, the substrate 11 with the p- and n-side electrodes20, 21 formed thereon is cleaved to have a resonant cavity length ofabout 500 μm. Then, the emissive and reflective end facets of theresonant cavity are coated appropriately. Finally, each of the laserdevices cleaved is mounted facedown onto a heat sink such that therespective upper surfaces of the electrodes formed on the laser deviceface the mount surface of the heat sink.

As to the semiconductor light-emitting device obtained in this manner,we observed the surface morphology at the crystal planes of theas-formed multiple quantum well active layer 15 using a scanningelectron microscope and an atomic force microscope. As a result, weconfirmed that the density of pits in the multiple quantum well activelayer 15 was lower than that of the pits observed in the conventionalGaInN/GaInN multiple quantum well layer by a couple of orders ofmagnitudes.

As described above, in the fabrication process according to thisembodiment, TEG is used as a gallium source when the multiple quantumwell active layer 15 is formed. Thus, compared to using TMG, the growthrate for the (0001) plane is not so different from the growth rate forthe {1-101} planes. As a result, it is possible to prevent the pits,which have been created on the faces of the crystal growing, fromexpanding.

In this embodiment, the well layers 151 are made of GaInN and thebarrier layers 152 are made of AlGaN in the multiple quantum well activelayer 15. Alternatively, the well and barrier layers 151 and 152 may bemade of any other mixed crystals, e.g., AlGaInN. Even so, the creationof the pits can also be suppressed effectively.

Similar effects are attainable even by the use of a nitridesemiconductor including boron (B) as an additional Group III element andarsenic (As) or phosphorus (P) as an additional Group V element, as wellas nitrogen (N).

Also, in the foregoing embodiment, the pressure inside the reactionchamber is set at about 800×10² Pa, which is slightly lower than oneatmospheric pressure (=1013×10² Pa), in the MOVPE crystal-growingprocess. Alternatively, the reaction pressure may be set at anyarbitrary value, because the present invention is not dependent on thereaction pressure.

Also, if the compositions and thicknesses of the well and barrier layers151 and 152 in the multiple quantum well active layer 15 areappropriately selected, then the multiple quantum well active layer 15as a whole can have no strain at all. This is because a compressivestrain induced in the well layers 151 can be completely canceled by atensile strain induced in the barrier layers 152.

In a multiple quantum well active layer 15 shown in FIG. 6 as a firstmodified example of this embodiment, the Al mole fraction of first andsecond barrier layers 152 may be set to 0.02, for example, and a thirdbarrier layer 152A with an Al mole fraction of 0.07, for example, may beprovided between the protective layer 153, which is a p-type conductivelayer, and the third well layer 151. In such a case, the energy Ec ofthe third barrier layer 152A at the lower edge of the conduction band ishigher than that of the second barrier layer 152 as shown in the banddiagram in FIG. 7. The heterobarrier formed by the third barrier layer152A prevents electrons externally injected from going over the welllayer 151 and overflowing into the p-type conductive layer. As a result,the electrons can be injected into the well layers 151 more efficiently.

Moreover, according to a second modified example, the Al mole fractionof the third barrier layer 152 may be gradually increased from 0.02 in apart thereof closer to the well layer 151 toward 0.07 in another partthereof closer to the p-type protective layer as shown in FIG. 6. Insuch a case, the electrons can also be injected into the well layers 151more efficiently. In addition, the energy Ev of the third barrier layer152A at the upper edge of the valence band is relatively low in a partthereof closer to the protective layer 153 and relatively high inanother part thereof closer to the well layer 151 as shown in the banddiagram in FIG. 8. Thus, the probability of holes in the third barrierlayer 152 can be reduced, and therefore, holes can be injected into thewell layer 151 more efficiently.

As described above, according to this embodiment, the creation of pitsin the multiple quantum well active layer 15 can be suppressed andelectrons and holes can be injected into the multiple quantum wellactive layer 15 more efficiently. Accordingly, a semiconductor laserdevice, which oscillates at a lower threshold value and is highlyreliable in term of its lifetime, can be obtained and is applicable as alight-emitting device to an optical disk apparatus.

Also, the present invention is applicable not only to a semiconductorlight-emitting device but also to a high-mobility electronic device suchas a heterojunction field effect transistor with a similarheterojunction to that of the inventive structure. Even then, thecreation of pits in the heterojunction is also suppressible, thusincreasing the mobility of electrons.

Embodiment 2

Hereinafter, a second embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 9 illustrates a schematic cross-sectional structure of asemiconductor light-emitting device according to the second embodimentof the present invention. In FIG. 9, the same members as thoseillustrated in FIG. 1 are identified by the same reference numerals, andthe description thereof will be omitted herein. In the semiconductorlight-emitting device according to this embodiment shown in FIG. 9, ann-type optical guide layer 24 formed on the n-type cladding layer 13 ismade of n-type GaN. A multiple quantum well active layer 25 located onthe n-type optical guide layer 24 is formed by alternately stackingGaInN well layers and GaN or AlGaN barrier layers one upon the other.And a p-type optical guide layer 26 located on the multiple quantum wellactive layer 25 is made of p-type GaN.

FIG. 10 illustrates a detailed cross-sectional structure of the multiplequantum well active layer 25 according to this embodiment. As shown inFIG. 10, three GaInN well layers 251, each of which is about 3 nm thickand has an In mole fraction of 0.08; first, second and third barrierlayers 252, 252 and 252A, each of which alternates with the well layers251 and is about 5 nm thick; and one protective layer 253, which isformed on the upper surface of the third barrier layer 252A, is about 5nm thick and has an Al mole fraction of 0.15 are stacked in this orderon the substrate.

In this case, the first and second barrier layers 252 are made of GaN,for example, and the third barrier layer 252A is made of AlGaN with anAl mole fraction of 0.04, for example.

The third barrier layer 252A with an Al mole fraction smaller than thatof the protective layer 253 is provided between the protective layer 253and the third well layer 251. Thus, as shown in the band diagram in FIG.11, a heterobarrier higher than those of the first and second barrierlayers 252 is formed by that. Al contained in the third barrier layer252A. Accordingly, it is possible to prevent the externally injectedelectrons from going over the well layer 251 and overflowing. Inaddition, piezoelectric field is also induced in such a direction assuppressing the overflow of electrons, thus injecting the electrons intothe well layers 251 more efficiently.

According to a modified example, the Al mole fraction of the thirdbarrier layer 252A may be gradually increased from 0 in a part thereofcloser to the well layer 251 toward 0.04 in another part thereof closerto the protective layer 253. In such a case, the electrons can also beinjected into the well layers 251 more efficiently. In addition, theprobability of holes in the third barrier layer 252A can be reduced, andtherefore, holes can be injected into the well layers 251 moreefficiently.

Embodiment 3

Hereinafter, a third embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 12 schematically illustrates an arrangement of an optical diskapparatus according to the third embodiment of the present invention.The optical disk apparatus according to this embodiment uses theinventive semiconductor light-emitting device as a light source sectionthereof. As shown in FIG. 12, the semiconductor laser device 41, whichincludes a laser chip packaged in a can, is disposed at such a positionthat the emissive end facet thereof faces the data-retaining side of anoptical disk 50, i.e., a storage medium on which desired data has beenrecorded. And a condensing optical system 40A is provided between thesemiconductor laser device 41 and the optical disk 50 in this opticaldisk apparatus.

The condensing optical system 40A includes: a collimator lens 42 forcollimating outgoing radiation 51, which has been emitted from thesemiconductor laser device 41, into parallel light; a diffractiongrating 43 for splitting the parallel light into three beams (notshown); a half prism 44 for transmitting the outgoing radiation 51 andchanging the optical path of light 52 that has been reflected from theoptical disk 50; and a condenser lens 45 for condensing these threebeams onto the optical disk 50. These members are placed in this ordersuch that the collimator lens 42 is closest to the semiconductor laserdevice 41. In the illustrated embodiment, laser radiation with awavelength of about 405 nm is used as the outgoing radiation 51.

Each of the three beams is condensed on the optical disk 50 as a spotwith a diameter of about 0.8 μm. A drive circuit 46 is further providedto correct a radial displacement of the optical disk 50, which isdetected based on the locations of these three spots, by moving thecondenser lens 45 appropriately.

On the optical path of the reflected light 52 outgoing from the halfprism 44, provided are a receiving lens 47 for converging the reflectedlight 52, a cylindrical lens 48 for detecting a focus error, and aphotodiode 49 for converting the condensed reflected light 52 intoelectrical signals.

As described above, the optical disk apparatus includes: the condensingoptical system 40A for guiding the outgoing radiation 51 emitted fromthe semiconductor laser device 41 onto the optical disk 50; and thephotodiode 49 receiving the light 52 that has been reflected from theoptical disk 50. If the inventive semiconductor light-emitting devicethat can emit blue laser radiation stably just as designed is applied tothis optical disk apparatus, then data that has been recorded on theoptical disk 50 at a high density can be read out (reproduced).

The laser chip preferably exhibits self-oscillating properties to readout information more accurately. This is because even if the opticaloutput power of the laser chip is relatively low, the laser chip is lesssusceptible to the effects of returning radiation in such a situationand the SNR improves as a result. When the laser chip is provided withsuch self-oscillating properties, there is no need to additionallyprovide any radio frequency circuit to minimize the effects of thatlight returning to the semiconductor laser device 41, thus simplifyingthe device construction advantageously and making it easier to downsizethe device.

Furthermore, the optical disk apparatus according to this embodiment canalso operate to output laser radiation at a power of as high as about 25mW. Thus, the optical disk apparatus can also write, or record, dataonto the optical disk 50. That is to say, the optical disk apparatus canperform both recording and reproducing operations alike using a singlesemiconductor laser device 41 and the performance thereof is excellentin spite of its simplified configuration.

Embodiment 4

Hereinafter, a fourth embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 13 schematically illustrates an arrangement of an optical diskapparatus according to the fourth embodiment of the present invention.The optical disk apparatus according to this embodiment uses theinventive semiconductor light-emitting device as a light source sectionthereof. Also, the optical disk apparatus can be of smaller size andthickness by providing a laser chip, a photodiode for detecting anoptical signal and a micro mirror for changing the optical path of laserradiation emitted from the laser chip on a single support member orsubstrate of silicon (Si). In the following description, the laser chip,photodiode and micro mirror will be collectively referred to as a “laserunit”.

As shown in FIG. 13, the laser unit 61 is disposed at such a positionthat the emissive end facet thereof faces the data-retaining side of anoptical disk 50, i.e., a storage medium on which desired data has beenrecorded. And a condensing optical system 40B is provided between thelaser unit 61 and the optical disk 50 in this optical disk apparatus.

The condensing optical system 40B includes a hologram 62 including: agrating pattern on a first incidence plane thereof to split the outgoingradiation 51 incident on the first plane into three beams; and aholographic pattern on a second incidence plane, on which the light 52that has been reflected from the optical disk 50 is incident, such thatthe light 52 is diffracted, condensed and diffused as ±first-order lightin the direction parallel to the surface of the disk 50. The system 40Bfurther includes: a quarter-wave plate 63 for converting linearlypolarized light into circularly polarized light, or vice versa; and anobjective lens 64 for condensing the outgoing radiation 51 onto adesired information track on the optical disk 50. These members areprovided in this order such that the hologram 62 is the closest to thelaser unit 61. An actuator 65 is further provided beside the condensingoptical system 40B to correct the deviation between the outgoingradiation 51 and the reflected light 52.

FIG. 14 illustrates a configuration for the laser unit 61 according tothis embodiment. As shown in FIG. 14, the laser unit 61 is formed on asingle substrate 71 of Si. A concave portion 71 a is provided in theprincipal surface of the substrate 71, and the inventive semiconductorlaser chip 72 is bonded onto the bottom of the concave portion 71 a withsolder, for example. A micro mirror 73 is provided on a side-wall of theconcave portion 71 a so as to face the emissive end facet of the laserchip 72 and to form an angle of 45 degrees with the principal surface ofthe substrate 71. In this arrangement, the outgoing radiation 51 emittedfrom the laser chip 72 is reflected from the micro mirror 73 to advancesubstantially vertically to the principal surface of the substrate 71.In this case, the micro mirror 73 is preferably a (111) plane of Si. TheSi (111) plane can be optically planarized easily because the (111)plane can be formed with ease by anisotropic etching and is chemicallystable. An angle of 54 degrees sharp is formed between the (111) planeand a (100) plane. Thus, if a substrate with its principal planeinclined from a (100) plane toward the [110] direction by 9 degrees isused as the substrate 71, then the sidewall forming the angle of 45degrees with the principal plane of the substrate 71 can be obtained asintended.

An output-monitoring photodiode 74 for monitoring the output power ofthe laser chip 72 based on the laser radiation emitted in a smallquantity from the reflective end facet of the laser chip 72 is formed onanother sidewall of the concave portion 71 a of the substrate 71 so asto form an angle of 63 degrees with the principal surface of thesubstrate 71 and to face the micro mirror 73. The surface of the micromirror 73 may be either bare silicon or be coated with a metal thin filmof Au, Ag or Al, which reflects the laser radiation at a highreflectance and absorbs the radiation at a low absorbance, to improvethe luminous efficacy of the laser radiation.

First and second photodiodes 75A and 75B are provided as photodetectorsfor receiving the reflected light 52 in the upper part of the substrate71, which is a bulk of a semiconductor, so as to be parallel to thereflective plane of the micro mirror 73 and to interpose the micromirror 73 therebetween. Each of these photodiodes 75A and 75B is dividedinto five portions extending in the direction parallel to the reflectiveplane of the micro mirror 73.

FIG. 15 illustrates a cross-sectional structure and the action of thehologram 62. As described above, a grating pattern 62 g is formed on thefirst incidence plane 62 a, on which the outgoing radiation 51 that hasbeen substantially emitted from a position 73 a on the micro mirror 73is incident. And a holographic pattern 62 h is formed on the secondincidence plane 62 b, which faces the first incidence plane 62 a andreceives the reflected light 52. First diffracted light 52 a of thereflected light 52 that has been incident on the hologram 62 and thendiffracted toward the first photodiode 75A is a beam focused in front ofthe light-receiving plane of the first photodiode 75A. On the otherhand, second diffracted light 52 b that has been diffracted toward thesecond photodiode 75B is a beam focused behind the light-receiving planeof the second photodiode 75B.

FIG. 16(a) schematically illustrates a planar layout of the laser unitaccording to this embodiment. In FIG. 16(a), the same components asthose illustrated in FIG. 14 are identified by the same referencenumerals. As shown in FIG. 16(a), the light-receiving area of each ofthe photodiodes 75A and 75B is divided into five areas. One of the threereflected (or diffracted) beams 52 is incident on the inner three areasof the five. And the other two reflected (or diffracted) beams 52 areincident on the remaining two outer areas.

Hereinafter, respective methods for detecting tracking error signal,focus error signal and information signal recorded on the optical diskwill be outlined.

The tracking error signal TES is given by the following Equation (1):

TES=(T 1−T 2)+(T 3−T 4)  (1)

where T1 and T2 represent the signal intensities of the beams incidenton the two outer areas of the first photodiode 75A and T3 and T4represent the signal intensities of the beams incident on the two outerareas of the second photodiode 75B, respectively, as shown in FIG.16(a).

The focus error signal FES is given by the following Equation (2):

FES=(S 1 +S 3 +S 5)−(S 2 +S 4 +S 6)  (2)

where S1, S2 and S3 represent the signal intensities of the beamincident on the three inner areas of the first photodiode 75A and S4, S5and S6 represent the signal intensities of the beam incident on thethree inner areas of the second photodiode 75B, respectively, as shownin FIG. 16(b).

The actuator 65 shown in FIG. 13 is driven such that the result ofEquation (2) becomes zero, thereby making the objective lens 64 followup the information tracks on the optical disk 50.

In the illustrated embodiment, FIG. 16(b) shows a situation where FES=0,thus indicating that no focus error is caused. In contrast, FES is notequal to zero in either situation shown in FIG. 16(c) or 16(d), thusindicating that a focus error has been caused.

The information signal RFS is also given in the same way by thefollowing Equation (3):

RFS=(S 1 +S 3 +S 5)−(S 2 +S 4 +S 6)  (3)

According to this embodiment, the optical disk apparatus can bedownsized and thinned by using the laser unit 61 shown in FIG. 14. Also,in fabricating the optical disk apparatus according to this embodiment,the assembly process is completed by simply performing the steps of:preparing the Si substrate 71 with the photodiodes 75A and 75B and theconcave portion 71 a formed in the principal surface thereof and withthe micro mirror 73 formed on a sidewall of the concave portion 71 a;and bonding the laser chip 72 onto the bottom of the concave portion 71a of the substrate 71. Accordingly, its fabricating process is alsosimplified and yet the production yield can be increased.

What is claimed is:
 1. A semiconductor light-emitting device of GroupIII-V compound semiconductors, the device comprising: a quantum welllayer, which is formed over a substrate and includes a barrier layer anda well layer that are alternately stacked one upon the other, a band gapof the well layer being narrower than a band gap of the barrier layer,wherein the well layer contains indium and nitrogen, while the barrierlayer contains aluminum and nitrogen, wherein a plurality of the barrierlayers are provided between p- and n-type conductive layers, and whereinone of the barrier layers that is in contact with the p-type conductivelayer has an aluminum mole fraction larger than that of the otherbarrier layer(s) that is/are not in contact with the p-type contactlayer and the aluminum mole fraction of the one barrier layer increasesfrom a part thereof closest to the n-type conductive layer towardanother part thereof closest to the p-type conductive layer.
 2. Asemiconductor light-emitting device of Group III-V compoundsemiconductors, the device comprising: a quantum well layer, which isformed over a substrate and includes a barrier layer and a well layerthat are alternately stacked one upon the other, the band gap of thewell layer being narrower than that of the barrier layer, first andsecond optical guide layers, the first optical guide layer beingprovided on one side of the quantum well layer that is closer to thesubstrate, the second optical guide layer being provided on another sideof the quantum well layer that is opposite to the substrate, wherein theband gap of the barrier layer is smaller than or equal to that of thefirst and second optical guide layers, and wherein the well layercontains indium and nitrogen, while the barrier layer contains aluminumand nitrogen.
 3. The semiconductor light-emitting device of claim 2,wherein the first or second optical guide layer contains aluminum andnitrogen.
 4. A semiconductor light-emitting device of Group III-Vcompound semiconductors, the device comprising: a quantum well layer,which is formed over a substrate and includes a barrier layer and a welllayer that are alternately stacked one upon the other, the band gap ofthe well layer being narrower than that of the barrier layer, first andsecond optical guide layers, the first optical guide layer beingprovided on one side of the quantum well layer that is closer to thesubstrate, the second optical guide layer being provided on another sideof the quantum well layer that is opposite to the substrate, wherein thebarrier layer has a strain vector of a sign opposite to that of a strainvector of the well layer, wherein the well layer contains indium and thebarrier layer contains aluminum, and wherein the band gap of the barrierlayer is smaller than or equal to that of the first and second opticalguide layers.
 5. The semiconductor light-emitting device of claim 4,wherein the first or second optical guide layer contains aluminum.